Last week, officials said the announcement had been delayed because Mr Bush wanted to announce it personally after the Republican Convention in Houston.
Renewed confrontation with Iraq is likely to be popular with the US electorate. A poll, by CNN/USA Today, last week found 62 per cent of voters saying that if Saddam Hussein continued to defy the United Nations the US should overthrow him. Only 14 per cent said they wanted no military action.
Despite reports of reservations in Arab states about the air exclusion zone and the possible disintegration of Iraq, it will be difficult for the US administration to do nothing without giving an impression of feebleness.
The US shift towards supporting the Iraqi Shias - previously regarded with suspicion in Washington as being under Iranian influence - was co-ordinated with Saudi Arabia over the past month. It would therefore be surprising if Saudi Arabia refused to grant any military facilities.
The danger for the Iraqi Shias is that allied over-flights would increase government repression, but without loosening President Saddam's authority. Most of the casualties in the Iraq marshes have been caused by artillery fire rather than by helicopter gunships, and there is no intention at the moment to attack the Iraqi gun batteries.
The British attitude is that the air exclusion zone in the south should operate very similarly to that over Kurdistan north of the 36th parallel which has been in place for more than a year. The difference between the two is that the Kurds had a well-established and organised resistance movement which does not exist among the Shias.
The breakup of Iraq, now mentioned with increasing frequency, is more complicated than it looks. While the three Kurdish provinces of Aibal, Sulaymaniyah and Dihok are homogeneous, Kurds are unlikely to abandon their claim to Dihok and the oilfields surrounding it.
Disentangling the Iraqi Sunni and Shia Muslims would be even more difficult. The largest concentration of Shias is in the capital Baghdad. The Shia working-class suburb of Saddam City in north-east Baghdad alone has a population of 750,000.